Magnetic resonance imaging (MRI) is used to produce high resolution images of the interior of the body (e.g., humans, animals, etc.) for such purposes of medical research, medical diagnosis, etc. The images are produced based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum.
Typically, magnetic resonance imaging is performed by placing a patient in a constant magnetic field, B0. A radio frequency excitation pulse (B1 field) is then transmitted into the patient. The excitation pulses cause magnetic moments of hydrogen nuclei to absorb energy. Upon removal of the excitation pulses, the nuclear moments begin to emit their absorbed energy and realign with the constant magnetic field, B0. During this realignment period, the nuclear moments emit radio frequency signals characteristic of the magnetic field and of the particular chemical environment in which the nuclei exist.
An RF coil may be used to both transmit the excitation pulses and receive the signals from the nuclei. Alternatively, one RF coil may be used to transmit the excitation pulses and another separate coil to receive the signals from the nuclei.
Multi-element radiofrequency resonator designs provide superior sensitivity for anatomical structures near the surface of the body and have utility for parallel imaging applications. One type of multi-element radiofrequency resonator design is known as a two-dimensional ladder network resonator. The general problem of two-dimensional ladder network resonators has been shown to be closely related to the problem of a vibrating mechanical membrane with suitable boundary conditions. This problem is most easily solved by writing down a recursion relation for Kirchhoff's voltage equations on the meshes of the structure of interest. The dispersion relation for the eigenvalues yields the frequency spectrum, while the eigenfunctions represent the mesh current amplitudes. From the eigenfunctions it is straightforward to calculate B1 maps.
U.S. Pat. Nos. 5,515,855 and 5,682,893 to Meyer et al. provide an example of low-pass two-dimensional ladder networks. The low-pass two-dimensional ladder networks provide a doubly degenerate homogeneous mode for circularly polarized magnetic resonance imaging applications. Further details can be found in the articles entitled “Two Dimensional Ladder Network Resonators,” Ballon et. al., Journal of Magnetic Resonance, Series A 111, 23-28 (1994) and “A 3×3 Mesh Two Dimensional Ladder Network Resonator for MRI of the Human Head,” Meyer et al., Journal of Magnetic Resonance, Series B 107, 19-24 (1995), the contents of which are incorporated herein by reference.
One of the primary disadvantages of the low-pass structures is that higher field applications of these structures are limited by the fact that the eigenvalue of the most homogeneous normal mode is lowest in frequency.